The present invention relates in general to wireline telecommunication communication systems, and is particularly directed to mechanism for automatically determining from a location such as a central office whether equipment installed at a remote end of a wireline link is span-powered, or is powered locally exclusive of the wireline link, and then applying a voltage to the wireline link that is appropriate for the identified type of remote termination equipment.
In addition to sending telecommunication signals, such as audio (e.g., voice), video and data over (copper) wirelines, service providers may also use the same set of copper lines to supply power (typically referred to as xe2x80x98loop poweringxe2x80x99) to a piece of telecommunication equipment (terminating device) that is installed at a location up to several miles away. While the power required may be very small or relatively large, depending upon device type, loop powering often requires the application of a relatively large voltage (e.g., on the order of 130 VDC) to the wireline link, and is used extensively on mid-span devices of digital dataphone service (DDS), integrated services digital network (ISDN), digital T1 transport, and high data rate digital subscriber loop (HDSL) networks.
For other types of telecommunication equipment, the terminating device is located inside the customer""s premises, and is powered xe2x80x98locallyxe2x80x99 from within the customer""s premises. Where the termination device is locally powered, there is no need to apply a very large voltage to the wireline link from the central office for powering the remote device; instead, it is only essential that the link carry a relatively small xe2x80x98sealing currentxe2x80x99xe2x80x94in order to prevent corrosion in the copper of the wireline loop. Operation of a termination device in the customer""s premises does not depend upon the presence of sealing current.
Due primarily to the continuing improved performance of telecommunication equipment, the distance at which the termination device may be located relative to the central office, and therefore the length of the connecting phone lines has increased. Because of this increase in copper loop length, the use of high voltage sources in the central office is necessary, in order to provide any significant amount of power to a remote termination device that is loop powered.
On the other hand, for the case of a very long copper loop to a remotely powered termination device, the voltage that must be applied to the loop for providing sealing current remains relatively low, since sealing current itself is a very small current. Moreover, it is an Underwriter""s Laboratory (UL) requirement that any voltage supplied from the central office and made available at a customer""s premises must be a xe2x80x98Safety Extra Low Voltagexe2x80x99 (SELV), the current specification for which is a voltage not to exceed a value on the order of 60 volts.
It will be appreciated therefore that the type of device installed in the telephone company""s central office for powering the (copper) wireline will depend on what type of termination device is located at the remote end of the loop. On the one hand a central office device having a SELV type voltage supply is needed for terminating devices that are locally powered; on the other hand, a central office device with a high voltage supply is needed for devices that are span powered. This creates two problems.
First, the telephone company must purchase and stock two types of central office loop powering devices. In addition to being more costly to the telephone company, using two different devices adds to the complexity of the overall communication system. Secondly, since the person installing the central office device cannot see what terminates the remote end of the copper loop, there is the possibility that a high voltage device will be installed at the central office, when in reality the remote end is terminated by locally powered customer""s premises equipment. Delivering a substantial loop powered voltage (e.g., on the order of 130 VDC) to a locally powered remote termination device is prohibited.
In accordance with the present invention, this twofold problem is effectively obviated by a new and improved central office-installable scheme, that is operative to automatically determine whether a termination device installed at a remote end of a wireline link is span-powered, or is powered locally exclusive of the wireline link. Once this determination has been made, the invention is effective to apply a voltage to the wireline link that is appropriate for the determined type of termination device. This eliminates the need for two separate central office loop-powering devices. In the course of making this determination, nothing higher than an SELV is supplied to the wireline link, so as to comply with UL standards for locally powered customer premises equipment.
For this purpose, the present invention employs a loop powering switch that is controlled by a microprocessor to selectively apply to the wireline loop one of two loop-powering voltages: an SELV compliant +/xe2x88x9248 volts, readily available in most central offices, and +/xe2x88x92130 volts. The default connection is the +/xe2x88x9248 volt supply to ensure compliance with the UL code requirement referenced above. Via a differential amplifier coupled to a loop current resistor installed in the wireline loop, an amplified voltage representative of the magnitude of wireline current is coupled to a comparator to determine the presence of loop current. The output of the comparator is coupled to the control processor, which is programmed to execute a prescribed remote termination sensing and wireline powering routine.
Pursuant to this routine, the line is first checked to detect whether it is connected to any remote termination device. For this purpose, an SELV voltage is applied to the loop via the switch and the voltage across the current sense resistor is monitored for the presence of loop current. If there is no loop current, it is inferred that there is no remote termination, and the wireline remains default-coupled to the SELV voltage. Once loop current is detected, implying that the wireline is coupled to a remote termination device, the routine determines what type of remote termination is at the far end of the loop.
To this end, the connection with the SELV supply is interrupted, to reduce the voltage applied to the loop to zero volts and, using the digital communication line card, a digital communication connection over the loop with the remote termination is attempted. If there is a response from the remote termination device, it is inferred that the remote termination is locally powered, and the loop voltage is changed back to the SELV voltage, so as to provide sealing current to the loop. If there is no response, it is inferred that the remote termination is span-powered, and the span-powering voltage (130 V) is coupled through the switch to the wireline loop.
Once the voltage appropriate for the identified type of remote termination has been applied to the loop, a data connection may be established between the line card and the remote termination device. If there is a loss of power during the data connection, the routine described above is reinitiated. In addition, the quality of the digital signal is continuously monitored. In response to an unacceptable degradation in signal quality, the link is first checked for the presence of loop current. If loop current is present, the data connection is reestablished. If there is no loop current, however, the entire routine is reinitiated.